This invention relates to a receiving apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) transmission system for receiving a transmitted signal that is the result of adding a guard interval onto a signal obtained by IFFT (Inverse Fast Fourier Transform) processing and then transmitting the signal. More particularly, the invention relates to a receiving apparatus in an OFDM transmission system in which, even if a delayed wave in excess of the guard interval is generated, excellent reception can be performed by reducing interference between symbols and interference between carriers.
Frequency-selective fading ascribable to a multipath environment occurs in wideband wireless communications. An effective method of dealing with this is multicarrier modulation, which divides the transmission bandwidth into narrow bands (subcarriers) that do not undergo frequency-selective fading, and transmits the subcarriers in parallel. At present, specifications regarding digital TV and audio broadcasts (in Japan and Europe) and wireless LAN (IEEE 802.11a) are being standardized based upon OFDM transmission, which is one type of multicarrier modulation. An OFDM-based modulation scheme has been proposed for next-generation mobile communication systems as well.
FIG. 48A is a diagram useful in describing multicarrier transmission. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to quadrature modulators 3a to 3d via low-pass filters 2a to 2d, respectively. In FIG. 48A, the serial data is converted to parallel data comprising four symbols S1 to S4. Each symbol includes an in-phase component and a quadrature component. The quadrature modulators 3a to 3d subject each symbol to quadrature modulation by subcarriers having frequencies f1 to f4 illustrated in FIG. 48B, a combiner 4 combines the quadrature-modulated signals and a transmitter (not shown) up-converts the combined signal to a radio-frequency signal and then transmits the radio-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown in FIG. 48B, in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the subcarriers.
In FIG. 48A, the serial/parallel converter 1 converts serial data to parallel data of four symbols. In actuality, however, the serial/parallel converter 1 converts the serial data to N (e.g., 512 or 1024) items of parallel data and performs multicarrier transmission with N-number of subcarriers.
With the OFDM transmission scheme, frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth subcarrier of multicarrier transmission and a modulation band signal transmitted by an (n+1)th subcarrier. FIG. 49A is block diagram of a transmitting apparatus based upon the OFDM scheme. The apparatus includes a serial/parallel converter 5 for converting serial data to parallel data comprising M-number of symbols (I+jQ, which is a complex number). An IFFT (Inverse Fast Fourier Transform) 6, which is for the purpose of transmitting the M-number of symbols as subcarriers having a frequency spacing shown in FIG. 49B, applies an inverse fast Fourier transform to the frequency data to effect a conversion to time data. A guard-interval insertion unit 7 inserts a guard interval GI and inputs the real and imaginary parts to a quadrature modulator 9 through low-pass filters 8a, 8b. The quadrature modulator 9 subjects the input data to quadrature modulation, and a transmitter (not shown) up-converts the modulated signal to a radio-frequency signal. In accordance with OFDM transmission, a frequency placement of the kind shown in FIG. 49B becomes possible, thereby enabling an improvement in the efficiency with which frequency is utilized.
FIG. 50 is a diagram useful in describing a serial-to-parallel conversion. A pilot P is time-division multiplexed ahead of each frame of transmit data. It should be noted that the pilot P can be dispersed within a frame in the manner shown in FIG. 51. If it is assumed that a common pilot per frame is composed of 4×M symbols and that the transmit data is composed of 28×M symbols, then the serial/parallel converter 5 will output M symbols of the pilot the first four times as parallel data and then will output M symbols of transmit data 28 times as parallel data. As a result, over the duration of one frame, a pilot can be transmitted four times upon being time-division multiplexed into all subcarriers. By performing a correlation operation between this pilot and an already known pilot on the receiving side, a channel can be estimated on a per-subcarrier basis and channel compensation can be carried out.
FIG. 52 is a diagram for describing the insertion of a guard interval. If an IFFT output signal conforming to M-number of subcarrier samples (=one OFDM symbol) is adopted as one unit, insertion of the guard interval signifies copying the tail-end portion of the signal to the leading end thereof. By inserting a guard interval GI, it is possible to eliminate the effects of intersymbol interference (ISI) caused by multipath.
FIGS. 53A and 53B are diagrams useful in describing interference between codes due to a delayed wave, in which reference characters A and B represent direct and delayed (reflected) waves, respectively. If delay time τ of the delayed wave B is less than a guard-interval length NGD, as shown in FIG. 53A, then a data symbol D0 of the direct wave A will not overlap another data symbol of the delayed wave B in a window timing W and, hence, intersymbol interference will not occur.
If the delay time τ of the delayed wave B is greater than the guard-interval length NGD, however, as shown in FIG. 53B, then the data symbol D0 of the direct wave A will overlap another data symbol D1 of the delayed wave B in the window timing W and ISI interference is produced as a result. Accordingly, the guard-interval length NGD is decided, taking into consideration a maximum delay time τmax of the delayed wave, in such a manner that ISI will not occur.
FIG. 54 is a block diagram illustrating a receiving apparatus in an OFDM transmission system. A radio receiver 11 applies frequency conversion processing to a received OFDM carrier signal, and a quadrature demodulator 12 subjects the receive signal to quadrature demodulation processing. A guard-interval removal unit 13 removes the guard interval GI from the receive signal after receive-signal synchronization is achieved. The resulting receive signal is input to a FFT (Fast Fourier Transform) unit 14. The latter executes FFT processing and converts the signal in the time domain to M-number of subcarrier signal (subcarrier sample) values S1 to SM at an FFT window timing.
A channel estimation unit 15 performs channel estimate subcarrier by subcarrier using pilot symbols time-division multiplexed on the transmitting side, and a channel compensation unit 16 multiplies the FFT outputs S1 to SM by respective ones of channel estimation values h1 to hM of each of the subcarriers. More specifically, using pilot signals, the channel estimation unit 15 estimates phase influence exp(jφ) and amplitude influence A ascribable to fading of each subcarrier, and the channel compensation unit 16 compensates for fading by multiplying the subcarrier signal components of transmit symbols by exp(−jφ) and 1/A. A parallel/serial converter 17 converts parallel data, which is output from the channel compensation unit 16, to serial data, and a data demodulator 18 demodulates the transmit data.
Thus, with OFDM, a guard interval GI is added onto one item of OFDM symbol data (referred to simply as “symbol data” below) and ISI will not occur even if a multipath delayed wave within the length of a GI symbol exists. This is advantageous in that demodulation can be performed without using equalization (i.e., such a system is immune to multipath fading).
On the other hand, adding on a GI symbol (a redundant symbol) causes a decline in transmission efficiency. In order to suppress this decline in transmission efficiency, the length of the OFDM symbol must be made large. This increases the number M of subcarriers in a fixed transmission bandwidth. This gives rise to certain problems encountered in multicarrier transmission, namely an increase in the ratio of peak-to-average power (degradation of performances ascribable to non-linear distortion in an amplifier) and a decline in fading tracking performance due to enlarged symbol length, and the number of subcarriers is designed in a tradeoff among these factors.
However, the delay time of a delayed wave along an actual transmission path varies greatly, and the delay spread is large, especially outdoors, e.g., 0.2 to 2.0 μs in urban areas and 10 to 20 μs in mountainous areas. The conceivable GI length usually cannot provide compensation for 100% of all service areas.
One solution to this problem is “An OFDM Receiving System for Multipath Environments of a Delay Profile Exceeding a Guard Interval” by Suyama, et al., Institution of Electronics, Information and Communication Engineers, Technical Report RCS 2001-175 (2001-11), pp. 45-50 (referred to as the “prior art” below).
With signal transmission for mobile radio according to OFDM, the transmission performance degrades markedly in multipath delayed propagation that exceeds the guard interval. The reason for this is ISI between OFDM symbols and intercarrier interference (ICI) within the same symbol. In order to suppress both ISI and ICI and improve the transmission performance, the prior art cited above consists of {circle around (1)} decision feedback equalization for removing the effects of ISI, {circle around (2)} maximum likelihood sequence estimation (MLSE) for removing the effects of ICI from the results of this processing and estimating a transmit-signal sequence, {circle around (3)} Fourier transform processing using a window function that is capable of reducing the number of states in maximum likelihood sequence estimation, and {circle around (4)} channel estimation processing by recursive least squares.
FIG. 55 is a block diagram of an OFDM receiver according to the prior art.
With this receiver, first an FFT window unit 50 subjects a receive signal to a Fourier transform within a rectangular window in a pilot interval for channel estimation and applies its output to a channel estimation unit 51. The latter performs channel estimation using a pilot. The FFT window unit 50 changes the window function of the data interval using the pilot. More specifically, (1) when the difference between multipath delay times falls within the guard interval, the usual rectangular window function is used, but (2) if a delay-time difference that exceeds the guard interval is observed, then a smooth window function such as a Hanning window is used in the data interval.
Next, the FFT window unit 50 performs a Fourier transform in the data interval using a window function. A subtractor 55 subtracts an ISI replica, which has been generated by an ISI replica generator 52, from the receive signal that has undergone the Fourier transform. This processing is executed en masse over all subcarriers. This processing section is referred to collectively as a decision feedback equalizer.
The above-mentioned receive signal from which the ISI has been eliminated is input to an MLSE (maximum likelihood sequence estimation) unit 53, which extracts a transmit-signal sequence of each carrier. This equalizer generates transmit-symbol candidates along the frequency axis of symbols at a certain time, and an ICI replica generator 54 generates a replica of the receive signal from the generated plurality of candidates. A sequence for which the square of the absolute value of the error between the receive signal and the ICI replica will be minimized is output as a transmit-signal sequence. The receiver of FIG. 55 also includes a parallel/serial converter 56, a serial/parallel converter 57, an arithmetic unit 59 and a squaring unit 60.
The prior art deals with the same topic as the present invention and presents the result of a simulation applied to a wireless LAN system. The goal of the prior art is to reduce ISI (intersymbol interference) and ICI (intercarrier interference) ascribable to a delayed wave that has exceeded a guard interval and its characterizing feature is to execute all processing in the frequency domain (namely with the circuitry that follows the FFT on the receiving side). Further, the receive FFT is subjected to time-domain filtering in order to suppress ICI, which spreads into the entirety of the band. Furthermore, MLSE is used for ICI removal and a Viterbi equalizer having M2 states (where M represents the number of states in M-ary modulation) will be required for each and every carrier. For example, in case of QPSK (M=4), there will be 16 states, and if the number N of carriers is 1024, then 1024 Viterbi equalizers will be needed.
Further, since the demodulated signal undergoes a hard decision by MLSE, soft-decision gain obtained if the signal is the conventional demodulated signal is not acquired at all when concurrent use is made of forward error correction (FEC). That is, the BER performance degrades. Furthermore, since MLSE is an exhaustive-search algorithm, the M-number of states in M-ary modulation that must be prepared is equivalent to the multiplier of the carriers observed (since there are two carriers in the case of the prior art, the number of states is the square). Accordingly, when concurrent use is made of an adaptive modulation scheme, which is adopted in the majority of present-day wireless systems (third-generation mobile communications and wireless LANs, etc.), the number of states that must be made available is equivalent to the maximum value of the number of M-ary modulation states, and MLSE itself must recognize the actual number of M-ary modulation states and must change over this number dynamically. Further, control is complicated with a multicarrier adaptive modulation system in which the modulation scheme is changed over subcarrier by subcarrier. Thus, problems still remain with the conventional method, namely the fact that computation cost for attaining the objective is high (especially in a case where concurrent use is made of adaptive modulation), and the fact that encoding gain declines when concurrent use is made of FEC.